The new horror film The Human Centipede tells the story of three unwitting tourists who are abducted by a crazy surgeon obsessed with creating exactly what the film's title suggests. This he proposes to do by attaching his three captives in a chain, with (look away now, squeamish people) the mouth (I'm serious, look away) of one (last chance!) surgically grafted on to the anus of the one in front.

There are two sorts of reaction to this. One is to be horrified, disgusted or showily contemptuous. The other is to find the idea, as I do, killingly funny. Despite being tagged as horror, films like this are really the province of people who have the second reaction rather than the first. Whether the idea is enough to sustain an entire movie is another issue. I suspect it's lousy. But I'm still going to see it.

Incidentally, I notice from the trailer that the boggle-eyed loon with the scalpel is German. That activates widely held prejudices. Only last week, it emerged that a German restaurant is seeking volunteers to donate their own flesh for cannibal gastronomy. If you claimed there was a reality TV show being filmed in Hamburg with the exact plot of The Human Centipede, compered by a Teutonic Davina in a rubber suit, most people would believe you.

But, German or not, the premise is funny, isn't it? Of all the ambitions you could have, making something as fiddly, ugly and manifestly useless as a human centipede is so absurd it's almost endearing. Apparently, when the surgeon tries to train his centipede (to fetch his paper, that sort of thing) it rebels. Later, it attacks him. He would have been much better off simply buying a labrador.

I remember – though only dimly, so forgive me for any error – hearing someone describe a script meeting for an episode of The Simpsons. They were writing a car chase and, for some reason, there was a wire strung across the road just above head height. One writer suggested it would be funny if Homer held his sandwich aloft, and the wire cut it neatly in two. Yes, agreed another writer, but it would be funnier if Homer held his sandwich aloft – and the wire chopped off his arm.

That's a blueprint for how this sort of horror works. The noise you make when someone goes "boo" is halfway between a yelp of fright and a yelp of laughter, just as the reaction to gross-out is a squirmily pleasurable mix of fascination and repulsion. In the most deadpan way, Tobe Hooper's 1974 Texas Chainsaw Massacre is a screamingly funny film – even down to the brief and unequal encounter between a maniac with a chainsaw and a guy in a wheelchair.

The laughter isn't incidental. The horror movie, in terms of its shape and dramatic mechanisms, is closer to farce than drama. If comedy is about unimportant things happening to unimportant people by chance, and tragedy about terrible things happening to important people in an inexorable way, then these films are comedies.

Schlock horror works on appalling twists of chance: taking the wrong turn in hillbilly country; picking up this hitchhiker, that sexy girl, or that alien embryo by mistake; stopping at the wrong motel; or, like Brad and Janet in The Rocky Horror Show, getting that darn flat tire outside Castle Von Bonkers. Centipede deploys the familiar scenario of the broken-down car.

There does exist a grim-inexorability school of horror movie: Michael Myers lumbering single-mindedly after Jamie Lee Curtis in the Halloween films, say. But even that series lost its grimness, the natural pull of horror films being towards the comic. Final Destination movies, which see Death methodically ticking latecomers off his list, are about as fatalistic as you can get – yet still are played wholly for laughs.

Lightness is all. If you felt deeply for the victims, the production-line cheerleaders and male models that Leatherface and his like dispatch by the vanload, you'd be appalled rather than entertained. But the chance wrong turns that lead to their demise still give a there-but-by-the-grace frisson to the viewer. You shrug when ditzy mitzi gets a meat hook through the neck – but you do finger your own clavicle thoughtfully afterwards. Perhaps this simply demonstrates that if the universe is a cruel joke, the least you can do is laugh.

In any case, it's best to think of horror films as cosmic slapstick, the pratfalls involving massive bodily disruption or death rather than a bump on the head. Instead of slipping on a banana skin, you get your head cut off. Instead of falling into a duck pond, you find both your knees broken and your mouth surgically attached to someone else's bumhole. The Human Centipede is descended not from Titus Andronicus, but from Laurel and Hardy.